linux-kernel - Re: [PATCH 1/2] perf_event: Fix incorrect range check on cpu number

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Date:	Tue, 15 Dec 2009 10:54:40 -0800
From:	Corey Ashford <cjashfor@...ux.vnet.ibm.com>
To:	Paul Mackerras <paulus@...ba.org>
CC:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Michael Neuling <mikey@...ling.org>
Subject: Re: [PATCH 1/2] perf_event: Fix incorrect range check on cpu number


Paul Mackerras wrote:
> On Tue, Dec 15, 2009 at 11:31:32AM +0100, Peter Zijlstra wrote:
> 
>> On Tue, 2009-12-15 at 19:40 +1100, Paul Mackerras wrote:
>>> It is quite legitimate for CPUs to be numbered sparsely, meaning that
>>> it possible for an online CPU to have a number which is greater than
>>> the total count of possible CPUs.
>>>
>>> Currently find_get_context() has a sanity check on the cpu number
>>> where it checks it against num_possible_cpus().  This test can fail
>>> for a legitimate cpu number if the cpu_possible_mask is sparsely
>>> populated.
>>>
>>> This fixes the problem by checking the CPU number against
>>> nr_cpumask_bits instead, since that is the appropriate check to ensure
>>> that the cpu number is same to pass to cpu_isset() subsequently.
>> Cute, do you actually have hardware that does this?
> 
> Yeah, Mikey ran across this on a POWER7 box here.

Does the perf tool need to be fixed too?  The "perf stat" tool, at least, has a 
"-a" switch that tells the tool to count the event on all cpus, and it does this 
by iterating over the number of cpus, 0..n, assuming they are all contiguous.

- Corey

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